The present invention is concerned with a brake pressure generator for a hydraulic brake system for use with automotive vehicles and more particularly, to such a system comprising a master brake cylinder actuable by at least one booster piston to which is applied a hydraulic auxiliary pressure in a booster chamber, and also by a pedal actuating mechanism. Such a system further comprises a control valve means that cooperates with the pedal actuating mechanism for controlling the auxiliary pressure.
Automotive vehicles having an elevated licensed weight require a brake pressure generator having a high boosting factor to maintain the pedal force and the brake pedal distance within the limits predetermined by the desired operating comfort. In a brake pressure generator of this type, an adequately high boosting factor is readily attainable by a suitable control pattern of the auxiliary pressure and by correspondingly dimensioning of the operating surface of the booster piston. However, in the event of a failure of the auxiliary pressure, the actuating forces obtainable by the pedal actuating mechanism will be hardly sufficient to generate the brake pressure required for an adequate deceleration of the automotive vehicle.
To overcome that disadvantage, in a brake pressure generator of the type described in co-pending U.S. application Ser. No. 758,306, filed July 24, 1985 for Braking Pressure Generator For A Hydraulic Brake System For Automotive Vehicles (corresponding to W. German Pat. application No. P 34 28 136.3), the booster piston, on the side facing the master brake cylinder, is provided with a section of smaller diameter to confine a slave cylinder chamber formed as an annular chamber which is reduced in size by the booster piston when applying the brake. The slave cylinder chamber is in communication with a valve controllable by the auxiliary pressure of the hydraulic force booster through which, in case of a failure of the auxiliary pressure, it can communicate with a non-pressurized intake reservoir and, in case of available auxiliary pressure, it can communicate with a working chamber of the master brake cylinder thereby delivering--with an auxiliary-pressure-supported application of the brake--additional pressure fluid from the slave cylinder chamber, through the master brake cylinder, into the brake system. In case of a failure of the auxiliary pressure, the slave cylinder chamber remains ineffective such that the actuation of the brake system, with a correspondingly extended brake pedal distance, is exclusively effected with the aid of the master brake cylinder. This brake pressure generator involves comparatively high efforts, as switching of the slave cylinder chamber requires a hydraulically drivable valve of complex construction. Moreover, it has proved to be disadvantageous in that connection of the slave cylinder chamber to the working chamber of the master brake cylinder involves a pressure impact detectable by the brake pedal.